This invention is related to the power generation industry and, more particularly, to the field of brushless exciters for power generators.
In the power generation industry, it is well understood by those skilled in the art that an exciter, e.g., a brushless exciter, is often used to start a power generator. As shown in the prior art of FIG. 1, a conventional brushless exciter 10 includes a diode wheel 12 having a wheel hub 14 thereof which is mounted to an exciter rotor shaft 15. The diode wheel 12 also has portions 13 thereof which are electrically insulated as illustrated. The diode wheel 12 has a heavy interference fit with the shaft 15 and a notch or positive stop 16 thereof when the wheel 12 is positioned on the shaft 15. Removal of the diode wheel 12 from the shaft 15, however, can be time consuming, difficult, and costly and can damage the windings, parallel rings, the shaft 15, the diode wheel 12 or other portions of the exciter in the process. Because of friction and heating during operation and the interference fit of the diode wheel 12 with the shaft 15, the metal materials of the shaft 15 and the wheel hub 14 of the diode wheel 12, e.g., both formed of forged alloy steel, tend to be held or bond together, especially with the heavy interference fit. Because of the heavy interference fit, enough temperature difference must be obtained between the diode wheel 12 and the exciter rotor shaft 15 to enable disassembly of the wheel 12 from the shaft 15. For example, the wheel 12 can be removed with high temperature heating to attempt to obtain the desired temperature differential, rapid quenching of the inside diameter of the wheel 12, and use of a high tonnage jack. Because of the thin construction of the rotor shaft 15, however, it is difficult to obtain the desired temperature differential without damage to various portions of the exciter 10 as described above.
In view of the foregoing, the present invention advantageously provides a thermally insulated diode wheel for a brushless exciter and methods of removing the diode wheel which substantially reduces any potential damage to various portions of the exciter. The present invention also advantageously provides a thermally insulated diode wheel for a brushless exciter and methods of removing the diode wheel which impedes heat transfer from a diode wheel mounted on an exciter rotor shaft. The present invention further advantageously provides a thermally insulated diode wheel for a brushless exciter and methods of removing the diode wheel which allows the diode wheel to be removed quicker, less costly, and without the necessity of having to use special machinery to remove the diode wheel from the exciter rotor shaft.
More particularly, the present invention provides an exciter which preferably includes an exciter rotor shaft, a thermal insulation layer mounted to the exciter shaft, and a diode wheel having a wheel hub. The wheel hub of the diode wheel is preferably mounted by an interference fit to overlie and abuttingly contact the thermal insulation layer positioned on the exciter rotor shaft. The thermal insulation layer is positioned on the exciter rotor shaft and between the shaft and the diode wheel in a location where electrical insulation is not necessary.
The present invention also provides a method of removing a diode wheel mounted by an interference fit to an exciter rotor shaft of an exciter. The method preferably includes heating a diode wheel positioned on an exciter rotor shaft having a thermal insulation layer positioned thereon to a desired temperature which enhances removal of the diode wheel from the shaft, quenching the exciter rotor shaft with a liquid coolant at a temperature less than air ambient temperature, delaying the heat transfer from the diode wheel to the exciter rotor shaft during the quenching of the exciter rotor shaft by the thermal insulation layer, and applying a force to the diode wheel to thereby remove the diode wheel from the shaft.
The present invention further provides a method of removing a diode wheel mounted by an interference fit to an exciter rotor shaft of a brushless exciter. The method preferably includes heating a diode wheel positioned on an exciter rotor shaft to a predetermined temperature which is less than a temperature which would otherwise substantially damage portions of the brushless exciter, delaying the heat transfer from the diode wheel to the exciter rotor shaft, and applying a force to the diode wheel to thereby remove the diode wheel from the shaft.
The exciter having a thermally insulated diode wheel and methods of removing a diode wheel from an exciter rotor shaft advantageously are particularly used in portions of exciters where there is no necessity to electrically isolate the wheel from the shaft. In other words, the problem discovered and addressed by the present invention is a need for thermal isolation. Also, the exciter having a thermally insulated diode wheel and methods of removing a diode wheel provide the benefits of being able to handle high compressive loads and yet still maintain a minimum interference fit of the wheel with the shaft during overspeed conditions.